A STUDY  OF  THE  COLORING  EFFECTS  OF  SMALL  AMOUNTS  OF 
RARE  EARTH  OXIDES  ON  FUSED  ALUMINA  AND  SILICA 


By 


WILLIAM  BULTMAN  HOLTON 


THESIS 


FOR  THE 


DEGREE  OF  BACHELOR  OF  SCIENCE 

IN 

CHEMISTRY 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 

UNIVERSITY  OF  ILLINOIS 
1921 


UNIVERSITY  OF  ILLINOIS 


G 


ui-i-iy. — yXr--^92-^ — 


THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

WILLIAM-  JiOJLTUIi 

ENTITLED A _S^UP_Y_ _0  JL _TKE _ _C. Q L QRUiG _ J3EJS3IIT il  _ QJ£  _ SJiALL. _Aw.Q LLU T5. _ HE __ 

RARE,  -EAR  Til  - -QaXDES  _ Oli  _ iTISED-  A.IAMIRA-  - AilD-  -SIX 1CA 

IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 
DEGREE  OF Baghslor  of  Saifinns  in  Chemistry 


HEAD  OF  DEPARTMENT  OF  ___ CHEiilSIEKJt 


Digitized  by  the  Internet  Archive 

in  2015 


https://archive.org/details/studyofcoloringeOOholt 


A STUDY  OF  THE  COLORING  EFFECTS  OF  SMALL 


AMOUNTS  OF  RARE  EARTH  OXIDES  UPON 
FUSED  ALUMINA  AND  SILICA 
Wm.  B.  Holton 

From  earliest  ages  man  has  admired  and  coveted  the  beautiful 
precious  stones  which  nature  has  given  to  us.  Many  have  tried  to 
imitate  them,  and  a few  attempts  have  been  made  to  produce  these 
precious  stones  synthetically.  Imitations  can  always  be  detected, 
and  synthesizing  has  met  with  success  in  only  two  cases.  Moissan 
has  made  microscopic  diamonds.  Verneuill  has  successfully  pro- 
duced large  size  rubies.  These  rubies  are  identical  with  natural 
rubies  in  all  their  physical  and  optical  properties,  with  one  ex- 
ception. This  difference  is  that  synthetic  rubies  contain  a vari- 
able number  of  spherical  gas  bubbles  of  microscopic  size,  while 
any  inclusion  of  foreign  matter  in  the  natural  ruby  has  followed 
the  lines  of  flow  and  hence  is  longitudinally  striated.  The  dif- 
ference does  not  detract  from  the  value  or  usefulness  of  the  syn- 
thetic ruby  and  may  only  be  detected  by  special  optical  instruments. 

The  present  work  was  undertaken  to  make  synthetic  rubies 
and  silica  gems,  then  to  study  the  theory  that  the  colors  in  our 
precious  stones  are  due  to  small  amounts  of  rare  earth  oxides. 

Procedure 

Verneuill’ s method  of  introducing  a very  finely  divided  mix- 
ture of  ammonium  alum  and  chrom-ammonium  alum  into  the  oxy-hydrogen 
blow-pipe  flame  produces  rubies  which  can  only  be  differentiated 
from  natural  rubies  in  the  manner  described  above.  Hoping  to  avoid 


. 


. 


. 


* 


■ 

. 


#2  WBH 


this  trouble  of  included  gas  bubbles,  the  fusions  were  made  in  an 
Arsem  Vacuum  Furnace  of  the  A-l  type.  Instead  of  using  the  alums, 
pure  alumina  was  prepared  from  ammonium  alum  beforehand.  Weighed 
amounts  of  alumina  or  silica  and  of  the  coloring  constituent  to  be 
used  were  ground  together  in  a smooth  stone  mortar.  This  well 
ground  material  was  placed  In  the  furnace  in  a crucible.  The  fur- 
nace was  sealed,  evacuated  and  then  heated  to  the  required  temp- 
erature. The  cooling  required  one  to  two  hours  time,  after  which 
the  vacuum  could  be  released.  The  observation  tube  was  then  re- 
moved, and  the  crucible,  containing  the  fused  charge,  was  withdrawn 
and  inspected. 

Apparatus 

The  furnace:  The  furnace  used  was  an  Arsem  Vacuum  furnace  of 

the  A-l  type,  i.  e.  the  small  vertical  type,  which  has  a heating 
element  about  6 in.  long,  supported  in  an  upright  position. 

This  heating  element  is  a graph?  + resistance  sector,  which  is 
made  in  the  form  of  a helix.  It  is  supported  at  both  ends  by  the 
electrode  leads,  which  are  made  of  seamless  brass  tubing.  Water, 
flowing  through  these  tubes  and  around  the  outer  shell,  serves  to 
keep  the  leads  and  container  cool.  The  helix  is  surrounded  by 
a hollow  cylinder  of  graphite  filled  with  graphite  powder.  This 
cylinder  acts  as  a radiation  screen  and  increases  greatly  the 
heating  efficiency  of  the  furnace.  The  crucible  is  placed  on  a 
pedestal,  which  arises  from  the  bottom  support  within  the  helix. 

This  whole  arrangement  is  secured  to  a heavy  stell  plate  properly 
insulated,  and  bolted  in  place  within  a steel  tank,  which  forms  the 
outer  shell.  For  further  details  of  the  furnace  see  the  bibliogra- 
phy and  Fig . 1 . 


. Ml 


. . 


' 


. 

• • 


#3  7/BH 


The  Construction  of  a Heating  Element:  The  helix  is  made 

from  a piece  of  Acheson  graphite  12"  long  and  2|"  in  diameter. 

Mark  the  end  centers  on  a centering  lathe.  Then  chuck  a 1-5/8"  . 

twist  drill  in  a lathe;  put  the  lathe  on  slow  speed  and  feed  the 
piece  of  graphite  against  the  drill  by  hand  pressure.  Feed  slowly 
and  keep  a firm  pressure.  Do  not  let  the  piece  wobble,  or  the 
hole  will  not  be  true.  It  will  be  necessary  to  drill  from  both 
ends  unless  an  exceptionally  long  drill  is  to  be  had.  Mount  the 
cylinder,  obtained  in  this  manner,  upon  a wooden  mandrel.  Then 
turn  down  in  a lathe  at  moderate  speed  to  the  dimensions  given 
in  the  drawing.  Fig.  2.  Then  adjust  the  spiral  gear  cutter  to 
give  2/3  of  a turn  to  the  inch  and  cut  the  thread.  Use  very  slow 
speed  (the  back-gears),  and  take  only  a small  cut  each  time. 

See  the  diagram  (Fig.  2)  for  width  of  thread  and  spacing  from  the 
ends.  Do  not  cut  the  thread  completely  through,  but  nearly  so. 

Then  slip  the  tube  from  the  mandrel.  Next  place  it  upon  a per- 
fectly flat  surface  and  carefully  finish  cutting  the  spiral  with 
a hack  saw  blade  held  in  the  hand.  Fit  the  ends  of  the  helix  to 
the  holding  cup  by  brushing  down  with  fine  sandpaper.  It  is  now 
ready  to  put  in  place  in  the  furnace.  See  Fig.  1. 

Evacuation:  The  evacuation  is  accomplished  by  means  of  a 

Geryck  Oil  Pump  connected  to  the  furnace  cylinder  by  heavy  pressure 
tubing.  The  line  is  connected  to  a monometer  to  determine  when 
evacuation  is  complete.  There  is  also  in  the  line,  a by-pass  valve 
for  the  purpose  of  releasing  the  vacuum  or  admitting  any  inert 
gas  to  the  furnace  cylinder.  The  rubber  tubing  and  ordinary  glass- 
stoppered  valves  prove  quite  satisfactory  on  the  vacuum  line. 

The  leakage  in  24  hours  changes  the  manometer  reading  by  6mm. 

Heating  and  Controlling  Apparatus:  The  heating  is  accomplishe< 


* 

* 


' 
* 


#4  WBH 


by  means  of  a 15  K.  W.  transformer.  The  primary  voltage  is  220 
Volts.  The  voltage  is  in  steps  of  10  volts  from  10  Jo  60  on  the 
secondary.  In  order  to  control  the  voltage  between  these  points, 
two  graphite  block  resistances  of  the  pressure  type  and  an  A.  C. 
ammeter  (0-75)  are  used  in  series  with  the  primary.  This  control 
group  may  be  shunted  by  closing  a heavy  switch. 

Measurement  of  Temperature:  The  temperature  may  be  read  in 

either  one  of  two  ways.  (1)  The  temperature  input-curve  is  obtain- 
ed for  the  furnace  by  melting  copper,  platinum  and  iridium.  A 
400  to  5 current  transformer  is  connected  to  one  of  the  feed  wires 
and  an  ammeter  (A.  C.  0-5)  is  connected  across  it.  A 0-30-60  volt- 
meter is  connected  directly  across  the  terminals  of  the  furnace. 

See  fig.  4.  The  metals  are  placed  within  the  furnace  and  the 
temperature  slowly  raised.  As  each  metal  softens  and  melts  down 
the  readings  on  the  meters  are  recorded  and.  from  these  the  power- 
input  in  K.  W.  can  be  calculated  and  plotted  against  the  melting 
point  for  each  metal.  A smooth  curve  is  drawn  through  these  points 
and  from  this  curve  the  temperature  at  any  future  time  may  be  read 
by  measuring  the  input.  (Note:  It  is  necessary  in  this  method 
that  the  temperature  of  the  crucible  be,  as  nearly  as  possible, 
the  same  as  that  of  the  helix.  (2)  The  other  method  of  reading 
the  temperature  is  to  use  directly  an  optical  pyrometer.  The  one 
used  was  a Leeds  and  Northrup  and  gave  very  good  checks  with  the 
temperatures  determined  from  the  power  curve.  It  checked  within 
25°  below  temperatures  of  1800°  C,  and  within  50°  for  temperatures 
around  2000°  C.  For  graphs  of  these  methods  see  Figs.  5 and  6 
respectively. 


#5  TO II 


Materials 

AlgOs:  Prepared  by  ignition  from  Mall inckrodt 1 s c . p . NH4AI ( SO4 ); 

C^203:  Prepared  by  ignition  from  Mali  inckrodt ' s c .p.NH/jCrC  804)2 

SiOg  : This  material  analyzed  98#  SiOg  and.  when  fused  alone 
gave  a clear  silica  bead.  It  was  therefore  considered  pure  enough 
for  the  work  to  be  undertaken. 

CeO:  Prepared  from  98#  Ce(NO^)g  by  ignition. 

MgO:  Baker's  analyzed  C.  P. 

Ndg03:  Obtained  from  rare  earths  laboratory  90-95#  purity. 

TiOg  : Obtained  from  rare  earths  laboratory,  97#  purity. 

Experimental 

Selection  of  a Crucible:  Graphite:  In  a small  piece  of 

Ache  son  graphite,  one  inch  in  diameter  and  one  inch  long,  a hole 
one-half  inch  in  diameter  was  drilled  to  a like  depth.  The  alumina 
mixture  was  placed  in  this  crucible  and  fired  in  the  furnace  to 
fusion.  The  fused  mass  when  examined  showed  just  enough  finely 
divided  graphite  to  give  it  a dirty  color.  Upon  using  this 
crucible  a second  time  the  amount  of  graphite  included  in  the 
fusion  was  less.  This  amount  gradually  diminished  with  repeated 
use,  but  never  became  negligible. 

CaO:  Next  a small  chunk  of  Cao  was  shaped  into  a crucible 
by  means  of  a knife  and  drill.  This  crucible  was  fired  to  2300°  C 
in  order  to  shrink  it.  After  shrinkage,  which  was  very  little, 
it  was  charged  with  a mixture  of  alumina  and  placed  within  the 
furnace.  When  a temperature  of  1800°  C had  been  obtained,  the 
alumina  reacted  with  the  CaO  and  fused  down  into  it,  eating  a hole 
right  through  the  CaO.  Therefore,  CaO  as  a crucible  was  out  of 
the  question. 

Molybdenum:  Next  a small  dish  of  pure  molybdenum  was  turned 


#6  WBH 


out  on  a lathe  and  filled  with  a charge.  The  crucible  was  placed 
in  the  furnace,  being  as  careful  as  possible  to  insulate  it  from 
any  graphite,  for  molybdenum  carbide  forms  and  melts  at  a temp- 
erature of  about  1750°  C,  while  molybdenum  melts  at  2535°  C.  MgO 
was  used  as  an  insulator,  but  no  cover  was  placed  over  the  dish. 
Upon  heating,  some  small  particles  of  graphite  disintegrated  from 
the  helix  and  dropped  into  the  dish.  When  a temperature  of  1800°  C 
was  reached,  the  dish  melted.  The  use  of  molybdenum  was  then  an 
impossibility,  unless  a means  could  be  devised  to  perfectly  in- 
sulate it  from  all  graphite. 

It  was  then  decided  to  use  graphite  crucibles  which  had  been 
fired  several  times. 


Results 


No. 

Mixture 

Temp. 

Color  Memoranda 

Atmosphere 

1 

Alg03 :3$Crg03 

2000°  C 

Fink-red 

Bubbly 

Vacuum 

2 

” b%  " 

tf 

Deep  ruby 

ft 

n 

3 

" . zi  " 

tt 

White 

Crystals 

Nitrogen 

4 

SiOgtO.5#  " 

1750°C 

Azure  blue 

SI  .bubbly 

Vacuum 

5 

" 1%  M 

it 

Blue -green 

bubbly 

tf 

6 

" 2% 

ft 

Dirty  green  ,T 

tt 

7 

AlgO^ : Vfo  CeO 

20000  C 

No  color 

SI  .bubbly 

tt 

8 

ft  pc/  H 

O/b 

tt 

if 

ti 

tt 

9 

SiOr> : 1%  CeO 

1750°  C 

Milky,  Opalescent  in 
spots. 

ti 

10 

" : Z>%  " 

tt 

More  densely  opalescent  n 

(bubbly) 

11 

" : 8%  " 

rt 

Opaque 

Bubbly 

tt 

12 

" :Ce2C5 

1800°C 

Brown -black 

lustreless  ” 

(No  bubbles) 

13 

A12°3:1$Nd2°3 

2000°C 

No  color 

Bubbly 

ti 

14 

" : 4%  " 

tf 

No  color 

Bubbly 

V 

15 

Si02:l^Nd203 

1750°C 

Paint  Pink- 

laverider  tint  n 

Oral e scent 

Bubbly 

H Hi  I jjl 

* 

. 


. 


■ 


#7  WBH 


16 

17 

18 
19 


Si02:4#Nd203  1750°C 

" :8%  " " 

SlOg : 5$FeO ; 3#CaO  M 
Al2C^5^Ti.02  1900°C 


Faint  Pink-lavender  more  Vacuum 
densely  opalescent  (bub) 

Same  color  Opaque  " 

Bubbly 

Black  flint  color  " 

No  bubbles 

Material  sparked  and  flashed  " 
violently 


Discussion 

Nos.  1 & 2 gave  a synthetic  ruby  which  differed  from  the 
natural  in  that  it  was  filled  with  a mass  of  minute  bubbles.  This 
mass  of  bubbles  could  be  removed  by  heating  to  a little  above 
the  fusion  temperature  and  cooling  very  carefully.  But  in  doing 
so  all  color  was  lost  in  alumina  fusions,  while  in  silica  fusions 
the  color  was  retained. 

No.  3 was  carried  on  in  an  atmosphere  of  nitrogen  to  eliminate 
the  bubbles  which  were  thought  to  be  due  to  the  vaporizing  of  the 
alumina  itself.  Instead  of  fusing,  the  alumina  reacted  with  the 
nitrogen  ar.d  gave  in  the  bottom  of  the  crucible  a growth  of  white 
fine  needle-like  crystals.  These  crystals  have  been  exposed  to 
aire  for  eight  weeks  and  are  perfectly  stable.  A test  was  made 
for  nitrogen  which  was  found  present.  It  was  thought  that  this 
might  be  some  form  of  aluminum  nitride.  Some  tests  were  made 
but  the  results  were  neither  positive  nor  negative.  Nitrogen  was 
used  with  silica  fusions  but  did  not  reduce  the  bubbles  so  the 
process  was  continued  in  vacuo. 

Nos.  4,  5,  Sc  6 gave  some  very  interesting  results  with  silica 
and  chromium  oxide.  The  No.  4 fusion  would  be  quite  pretty  if  it 
were  possible  to  eliminate  all  the  bubbles. 

Nos.  7,  8,  13  and  14  showed  that  CeO  and  Ndg03  have  no  effect 
on  alumina. 


Nos.  9,  10,  Sc  11  indicated  that  CeO  has  no  coloring  effect 
upon  silica,  but  produces  instead  an  opalescent  effect  upon  the 


#8  WBH 


surface,  when  used  in  very  small  amounts. 

No.  12  was  an  attempt  to  form  a cerium  cilicate  by  fusion. 
The  mass  obtained  was  not  analyzed. 

Nos.  15,  16  & 17  indicated  that  Nd203  faintly  colors  silica, 
and  produces  an  opalescence,  when  used  in  very  small  amounts. 

In  larger  amounts,  as  in  the  case  of  CeC,  the  mass  becomes  opaque. 
This  leads  one  to  believe  that  possibly  all  of  the  rare  earth 
oxides,  being  quite  heavy,  would  produce  the  same  effect. 

No.  18  looked  very  much  like  real  flint  and  was  as  hard. 

No.  19  Is  discussed  in  the  notes. 

Notes 


During  this  work  the  melting  points  of  some  refractories 
were  determined  which  compare  with  American  and  German  literature 


in  the  following  manner. 

Substance  M. 

P.  Observed 

American 

German 

MgO 

2500°  C 

1800°-  2000° 

2800° 

CaO 

2300°  C 

1950°  C 

2500° 

SrO 

2000°  C 

3000°  C 

White  Heat 

It  was  observed  that  it  was  necessary  to  heat  A1203  very 
gradually  up  to  a temperature  of  8000  c to  prevent  the  A1203  from 

violently  hopping  about  and  out  of  the  crucible.  This  effect  was 

) 

first  noticed  with  alumina  formed  by  decomposing  the  ammonium  alum 
over  a Meker  burner,  and  it  was  thought  possible  that  it  might  not 
have  been  completely  decomposed.  Therefore,  some  of  the  alumina 
was  fi^ed  in  a muffle  furnace  to  a temperature  of  1200°  C.  Upon 
using  this  material  the  effect  was  the  same,  when  heated  Rapidly 
in  an  atmosphere  of  nitrogen  and  no  such  hopping  about  would  occur 
This  leads  to  a possiblitly  of  two  conclusions. 


, 


. 


#9  WBH 


The  first  and  more  probable  is,  that  due  to  the  helix  form  of  the 
heating  element,  the  hopping  about  is  caused  by  magnetic  effects 
upon  alumina.  The  other  possibility  is  that  the  alumina  has 
adsorbed  moisture  so  that  when  heated  rapidly  in  vacuo  it  jumps 
about  because  of  the. rapid  loss  of  this  moisture,  where  as  it  would 
not  lose  this  moisture  so  rapidly  in  an  atmosphere  of  nitrogen  at- 
normal  pressure.  There  is  one  piece  of  evidence  which  substantiates 
the  first  theory.  If  a smooth  bottom  graphite  crucible  is  placed 
in  the  furnace  on  a support  with  a smooth  top,  and  heated  to  a 
temperature  of  2100°  C,  this  crucible,  after  being  held  at  this 
temperature  for  a short  time,  will  slide  across  the  pedestal  until 
it  touches  the  helix.  Also  a molten  charge  of  either  silica  or 
alumina  will  whirl  around  within  the  crucible,  in  the  direction 
of  the  spiral  cut  of  the  helix. 

It  was  observed  in  the  case  of  alumina  and  the  rare  earth 
oxides  that  at  a temperature  just  below  the  fusion  point  there 
was  considerable  sparking  and  flashing.  This  was  so  vigorous  in 
case  of  TiOg  (No.  19)  that  all  the  charge  was  exploded  from  the 
crucible.  It  may  be  this  same  manner  of  loss  is  the  cause  of  the 
lack  of  color  in  the  alumina  fusions  with  CeO  and  NdgCk. 

Conclusions 

1.  Molybdenum  would  make  the  most  suitable  crucible,  if  it 
be  perfectly  insulated  from  all  graphite. 

2.  Alumina  fused  with  a small  amount  of  CrgOg  gives  a 
product  which  has  the  color  and  hardness  of  a natural  ruby. 

3.  The  depth  of  color  imparted  to  the  alumina  is  a function 
of  the  amount  of  CrgC>3  added. 

4.o  The  oxides  CeO  and  Nd203  in  the  proportions  and  manner 
used  are  unsuitable  for  the  purpose  of  coloring  either  fused 


, 

♦ 

* 


, 


. - 

' 

. 


. 


#10  WBH 


alumina  or  silica. 

5.  The  difference  "between  the  action  of  Cr2(&  upon  fused 
alumina  and  fused  silica,  upon  continued  heating,  may  lead  to 
some  light  upon  the  state  in  which  the  Cr  or  Cr203  is  combined 
with  the  AI2O3  in  the  ruby. 


Bibliography 

Instruction  Book  No.  88714  General  Electric  Company. 
The  Production  and  Identification  of  Artificial  Precious 
Stones,  by  Noel  Heaton.  Smithsonian  Records  1911. 


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